1. Field of the Invention
The present invention concerns a device for seeking a connection between a terminal and a mobile radio system.
2. Description of the Prior Art
In mobile radio systems such as the GSM paneuropean digital mobile radio system, for example, a terminal communicates with a base station by way of communication channels conveying radio signals. These systems comprise a plurality of channels for transmissions from the terminals to the base stations or for transmissions from the base stations to the terminals.
These channels include a control channel which is transmitted continuously and which enables a terminal to access the system via the base station transmitting this channel in order to set up calls. The terminal must therefore identify this control channel in order to acquire the information enabling it to declare itself in the system. This information includes synchronization information and for this reason this procedure is usually called the synchronization procedure.
A two-stage method is usually adopted to achieve synchronization. In a first stage the terminal measures the power on all received channels. The terminal then attempts to synchronize with the channel received at the highest power. If it fails to do so it tries the other channels in decreasing received power order until it is able to synchronize. This method is that which is the subject of GSM Recommendations 4.08 version 3.11.0 and 5.08 version 3.7.0.
The synchronization procedure is carried out systematically whenever the terminal is switched on and also, and more generally, after any loss of synchronization, i.e. if the radio link carried in particular by the control channel between the base station and the terminal is interrupted. This interruption may be deliberate, for example switching off of the terminal, or accidental. The terminal may be temporarily unable to synchronize because of unsatisfactory radio reception conditions. This applies in a tunnel, for example, or more generally whenever it is in a shadow area in the sense in which this term is used in the radio art.
The synchronization procedure used in most cases is of particular benefit if the mobile radio system comprises a plurality of radio networks usually managed by different operators, each network covering a territory corresponding to all of a country or to one or more regions within a country. These radio networks are connected to the cable networks of the corresponding countries so that a mobile radio system terminal can communicate with a cable network terminal.
There may be several radio networks in the same country, their coverage areas overlapping partly or totally. Then, if a terminal wishes to access the mobile radio system, because there is a choice the question arises which radio network it will connect to. To this end the terminal includes a connection seeking device to establish a list of the radio networks to which it could connect.
A particular terminal which can operate throughout the mobile radio system is affiliated to a specific radio network, the home network, in just the same way as a telephone terminal is affiliated to a specific cable network. The reason for this is that the terminal is the means of obtaining services such as speech or data communication provided by the network operator who needs to know the identity of the terminal, in order to bill for the services provided if for no other reason.
It is desirable for priority to be given to connecting a terminal to its home network. The terminal user has an agreement with an operator who provides certain services that a different operator may not necessarily be able to supply; this applies to data communications, for example. Also, if a terminal connects to a radio network that it is convenient to refer to as a "visitor" network separate from its home network, this presupposes that the two operators concerned have an agreement authorizing such connection. This is not always the case, but assuming that it is, it is necessary for information to be exchanged between the two networks for the terminal's calls to be managed. The first item to come to mind is the cost of the service provided on the visitor network which must be passed on to the terminal's home network in order for the user to be billed accordingly.
It is therefore known for the connection seeking device of a terminal to be activated regularly either by manual action of the user or automatically when the terminal is connected to a visitor network of the same country as its home network, so that it breaks the connection with the visitor network to establish a new connection with the home network. This procedure is the subject matter of GSM Recommendation 03.22 version 4.0.0.
This connection seeking device uses the synchronization procedure previously defined because, before establishing whether it is possible to connect to a network, it is necessary to synchronize to a control channel transmitted by the network.
Generalizing these considerations, it follows that the connection seeking device is essential, whether the terminal is able to choose the network to which it can connect or not. The device comprises at least a synchronization unit to implement the synchronization procedure.
The synchronization unit is usually controlled by a time-delay unit. During an initial period of fixed duration the time-delay unit controls the synchronization unit continuously until synchronization is achieved on one control channel at least. If synchronization is not achieved at the end of this initial period the time-delay unit ceases to control the synchronization unit for a first waiting period before causing it to try again. If synchronization is still not achieved at the end of this first waiting period the time-delay unit initiates a second waiting period after which a further attempt is made. The time-delay unit therefore controls the synchronization unit periodically, the successive waiting periods between the various attempts at synchronization having the same duration. This solution is that adopted by the GSM in the specific case of home network seeking, the subject matter of Recommendation 02.11 version 4.2.0.
The connection seeking device naturally consumes power for as long a synchronization is not achieved, which reduces the operating time between battery recharging/replacement in the case of a terminal with a limited capacity battery.
This being so, European patent application EP 0 490 441 discloses a connection seeking device in which the time-delay unit comprises means for fixing the duration of the second waiting period so that it is longer than the first waiting period.
Thus as time passes the waiting periods get longer, the effect of which is to reduce power consumption. However, the longer waiting periods naturally tend to delay terminal synchronization, which is undesirable from the terminal user's point of view.
An object of the present invention is accordingly to achieve a compromise between reducing power consumption and increasing the availability of the terminal.